Generally, casting is the process by which molten material is formed into solid shapes. A known method for casting materials has involved the use of rolling cylinders to compress slabs of cast material to a desired thickness. However, this process is very energy intensive and costly.
An alternative casting method for producing a strip of material of a desired thickness, known as strip casting, incorporates a rotating wheel, drum, belt or other substrate. The rotating substrate is placed in close proximity to a casting nozzle from which molten material flows. The molten material is deposited on the rotating substrate where it cools, solidifies or "freezes", and is subsequently removed for further processing.
However, when the molten material is initially introduced through the casting nozzle and onto the casting wheel, heat is exchanged from the high temperature molten material to the lower temperature casting nozzle and casting wheel. This transfer of heat energy to the casting nozzle and the casting wheel causes them to expand, often in an unpredictable and non-uniform manner. As a result of this expansion, the distance between the adjacent surfaces of the casting wheel and the casting nozzle is often reduced.
Until the temperatures of the casting nozzle and the casting wheel reach a steady state, at which time further expansion of the casting nozzle and the casting wheel is minimized, the gap between them will not be a uniform or constant distance. In at least the case of planar flow casting, an example of which is illustrated in U.S. Pat. No. 4,771,820, the gap between the casting nozzle and the casting substrate can affect the thickness of the cast material, which is generally crucial to the quality of the cast material. If the cast material does not have the desired thickness, it may either be scrapped or mechanically reformed, both of which are expensive, time consuming, and inefficient.
The inability to control and maintain a desired distance or gap between the casting nozzle and the casting wheel can also cause a variety of other problems during casting. For example, if the distance between the casting nozzle and the casting wheel becomes too large, the molten material can flow along the face of the casting nozzle rather than onto the casting wheel. Material which is not deposited onto the casting wheel will inherently begin to cool as it flows along the nozzle, and can thereby interfere with the efficient operation of the machinery and compromise the quality and uniformity of the resultant cast product. Conversely, if a minimum gap between the casting nozzle and the casting wheel is not maintained, contact may occur between them which can result in severe damage to both the nozzle and the wheel. Such a situation obviously interferes with the safety and efficiency of the casting process.
In particular, when steel and other high temperature materials are strip cast, the relative expansions of the casting nozzle and casting wheel are virtually impossible to avoid. Since it is not generally economical to pre-heat a casting nozzle and a casting wheel to their steady state temperatures, a variety of methods have been used to measure and maintain the distance between a casting nozzle and a casting wheel. An example of an unique pneumatic device and method for monitoring and maintaining the distance between a casting nozzle and a surface of the casting wheel is disclosed in the commonly owned U.S. Patent Application entitled PNEUMATIC GAP SENSOR AND METHOD, filed concurrently herewith in the names of Karl T. Bagdal, Edward L. King, and Donald W. Follstaedt.
Some of the known methods include: product measurement, wherein the thickness of the cast material is measured downstream dynamically and the gap between the casting nozzle and casting wheel thereafter adjusted to compensate for measured thickness variations; and laser gap sensing, wherein a laser beam is utilized to measure the gap between the casting nozzle and the casting wheel.
All of the known methods and equipment have serious drawbacks, however. Indirect or downstream control of the distance between the casting nozzle and casting wheel from the downstream measurement of the resulting thickness of the cast material is complicated by the possible influence of other casting variables, such as casting speed, cooling, and composition, on the measured cast thickness. Moreover, downstream measurements are by definition "after the fact" quality controls, and undesirable rework or scrapping of the measured cast product is not avoided. Laser methods, on the other hand, are expensive and complicated to perform, especially for casts of wide strips of very hot alloys such as steel. In addition, lasers require a straight line of sight between the laser source and the photodiodes or similar laser detectors, through which the laser beam may travel. Unencumbered straight lines of sight are often not available between the expanding casting nozzle and substrate and, at least, difficult to provide. Moreover, the presence of smoke, heat, dust and other gases and particles produced during casting may interfere with (e.g. diffract) and restrict the passage of a laser light through the gap. Examples of devices of these types are disclosed in U.S. Pat. No. 2,383,310 and U.S. Pat. No. 4,399,861.
Consequently, heretofore, there has not been available a simple, reliable and economical device for maintaining a predetermined gap between two surfaces. In particular, such a device has not been available for use in hostile environments such as strip casting.